Improve error messages for database object stats manipulation functions during recovery

Hi,

When database object stats manipulation functions like pg_set_relation_stats() are run,
they currently produce the following error and hint messages, which are "internal"
and make it hard for users to understand the issue:

       ERROR:  cannot acquire lock mode ShareUpdateExclusiveLock on database objects while recovery is in progress
       HINT:  Only RowExclusiveLock or less can be acquired on database objects during recovery.

So I'd like to propose updating these to clearer messages:

       ERROR:  recovery is in progress
       HINT:  Database object statistics manipulation functions cannot be executed during recovery.

Thought?

I've attached a patch implementing these changes. It also updates the documentation to
clearly state that these functions are not available during recovery.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION